preparation and properties of carbon materials and properties of carbon materials ... such as seal...

Preparation and Properties of Carbon Materialsfor mechanical applications

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PREPARATION AND PROPERTIES OF CARBON MATERIALS

Schunk Carbon Technology: Always at your side.

Schunk Carbon Technology focuses on development, manufacture andapplication of carbon and ceramic solutions. It combines innovativespirit and technological expertise with exceptional customer service toprovide a range of products and services unique to the market.

In Schunk Carbon Technology, you have a partner who can offer all the technological

possibilities of an international company and implement ideas custom-tailored to your

needs, both for high-volume industrial markets and for highly specialized niche markets.

A Schunk Group division

Enabling, idea-driven, cooperative –If you want to apply technology todevelop better products and capturenew markets, we can help.

The Schunk Group has been supporting

customers with innovative technologies since

1913. As an idea-driven technology company,

innovation is fundamental to our culture. We

forge long-lasting, cooperative working

relationships with our clients.

You will find our custom-tailored, high-tech

products and systems in markets such as; carbon

technology and ceramics, environment

simulation and air-conditioning technology,

sintered metal and ultrasonic welding. The

Schunk Group is active in a large number of key

industries, from automotive, rail, aviation and

marine technologies to solar and wind energy to

the chemical and machine production industries.

Our 8,000 employees in 29 countries are ready

to serve you.

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Carbon

The chemical element carbon occurs in two main ordered lattice structures: diamond and graphite. The properties of the two modifications could hardly be more different. While diamond is the hardest natural material and an insulator, graphite belongs to the softer materials and is electrically conductive.

The special lattice structure of graphite, a layer lattice,

ensures good sliding properties. While the atoms in a plane

are very strongly joined with each other through covalent

bonds, only van der Waals forces act between different

planes. Under a mechanical load, planes begin to slide

along each other.

Carbon atom

Van der Waals force(Weak bonding force)

Covalent bonding(Strong bonding force)



Technical carbon materials

In tribological applications, two groups of carbon graphite and electro graphite materials are widely used and are often the only technical solution. In addition to their excellent sliding properties, it is the mechanical features that distinguish these ceramic materials.

Carbon graphite and graphite materials are generally produ-

ced in polygranular and/or polycrystalline form. This means

that the raw material grains of such carbon materials are

composed of tiny crystallites with different orientations.

Because of this microcrystalline structure, the macroscopic

body often does not have the typical anisotropic crystalpro-

perties of the graphite crystal. The extreme anisotropy of

the electrical conductivity or coefficient of thermal expansi-

on is virtually non-existent, or at least greatly attenuated in

polycrystalline materials. The low anisotropy of the charac-

teristics that does however occur in polycrystalline carbon

materials is linked predominantly to the pressing process,

as well as the type of raw material. Thus, isostatic carbon

materials, for example, have no anisotropy, while one or

both sides of hydraulically pressed materials exhibit slightly

more pronounced anisotropy.

Resin bonded carbon graphite supplements the range

of materials for tribological applications on the polymer

side. These materials are distinguished by the low cost of

production in large quantities and the possibility of creating

complex forms.

On the side of the carbide ceramics, graphite-filled SiC

materials also bear mentioning. A special feature is certainly

the silicon carbide graphite composite material SiC30 from

Schunk.

Further technical carbon products can be produced using

carbon or graphite fibers. These can be produced, for

example, through thermal treatment of polymeric fibers,

mostly polyacrylonitrile (PAN). Carbon fibers are used for

reinforcing polymers (CFRP), carbon (CFC, C/C), ceramics

(CMC) and metals. These composites are used primarily

where the combination of high stiffness and strength with

low weight plays a crucial role. Well-known applications for

CFRP are sporting goods or components for the aerospace

industry, which do not undergo high temperature stress. For

high temperature applications, e.g. in the semiconductor

industry or furnace construction, C/C materials are used. As

non-brittle fracture, high-strength ceramics these materials

are increasingly attractive for use in tribologically stressed

components.

Furthermore, diamond and diamond-like (DLC) coatings are

gaining in importance in the tribological field. The incredibly

elaborate diamond coatings withstand adverse conditions,

even brief dry running, and in individual applications where

SiC and SiC-C composite materials cannot be used, there is

no other alternative.

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Manufacture of carbon graphite and graphite materials

The production of the materials is carried out in line with production methods that are based on classic ceramic technologies. At Schunk, some of this is achieved through fully automatic processes and monitored online.

Material processing and mixing

The melting point of carbon is above 4000 °C at a pressure

of 100 bar. Carbon sublimates at lower pressures. Thus,

technical carbon cannot be produced by simple sintering

processes. Therefore, the production of carbon graphite

and graphite materials occurs in a filler/binder system.

Raw materials such as petroleum cokes, pitch cokes, car-

bon blacks and graphites are milled to defined grain size

distributions. These fillers are then mixed, preferably on

twin-screw extruders at elevated temperature with a

thermoplastic binder. Alternative coal tar pitch or petrole-

um pitch and synthetic resins can be used for this purpose.

The mixture is then ground to a powder for the shaping

process.

Shaping

The ready to press mixtures are formed into so-called

green parts through unidirectional die moulding or isostatic

pressing.

Carbonizing parts

The green bodies are then carbonized. A variety of furnaces

with specific heating rates, maximum temperatures and

furnace atmospheres are used for this purpose, depending

on the material, dimensions and the desired material proper-

ties. During the carbonization process pyrolysis takes place,

i.e. decomposition of the binder into volatiles and carbon.

The volatiles produce an open pore structure. The binder

remains in the molded article as so-called binder coke and

ensures high strength and hardness. These materials are

referred to as carbon or carbon graphite.

Graphitization

Carbon graphites are partially amorphous and a little

graphitic. To produce graphite materials, carbon graphites

are graphitized at temperatures of up to 3000 °C. At Schunk

this is mainly achieved through the Acheson process. Here,

the material to be graphitized is packed between two furn-

ace electrodes and positioned as a resistor in the secondary

circuit of a transformer. The material is thus brought to the

graphitization temperature by resistance heating. Larger

graphitic areas are thus formed by recrystallization. Such

electrographites generally demonstrate good sliding proper-

ties, have low electrical resistance, high thermal conducti-

vity and improved corrosion resistance compared to carbon

graphites.

Impregnation

The porosity of carbon graphite and graphite materials

can vary in a wide rage. This porosity can be reduced or

even eliminated through impregnation. Many tribological

applications require impermeability to fluids; other targeted

material properties can also be influenced by the impreg-

nation medium. At Schunk, impregnation usually takes

place via a vacuum-pressure process. Impregnation can be

achieved with different resins, metals such as antimony or

copper and with inorganic salts. Densification with carbon is

also possible.


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Binder Raw material Crushing Milling Sieving

Mixing

Milling Homogenizing

Isostatic Molding Die Molding

Carbonization

Spec. Treatments Graphitizing Impregnating

Machining Inspection Machinedcarbon graphite and graphite parts

Inspection

Carbon graphiteblanks


Properties of carbon graphite and graphite materials

Porosity

The production-related porosity of carbon graphite and

graphite materials leads to a certain permeability to fluids.

For some uses the existing pores are not a problem. How-

ever, porous materials are unsuitable for sealing elements

such as seal rings for mechanical seals. The open porosity

of carbon graphite and graphite materials can be reduced/

completely closed by means of impregnation (see previous

section on „impregnation“). Micrographs of unimpregnated and impregnated material

Bulk density

Because of the existing pores, it is common to specify the

apparent density or bulk density. This can vary from 1.5 to

3.3 g/cm3 depending on initial porosity and impregnation.

Carbon components are extremely light.

Chemical resistance

Carbon graphite and graphite materials are distinguished

within the group of corrosion-resistant materials by their

excellent chemical resistance. For details, please see our

brochure 39.12 on chemical resistance.

Temperature resistance

In an oxygen atmosphere, carbon is oxidized at high tem-

peratures. In air, this oxidation occurs for carbon graphite

materials from about 350°C and for graphites from 500°C.

Through special finishing operations, the temperature

resistance of graphite in an oxidizing atmosphere can be

increased to over 600°C. In a non-oxidizing atmosphere,

the temperature resistance of carbon graphite and graphite

is determined by the treatment temperature during the

production process and thus is about 1000°C or > 2500°C.

For resin and metal impregnated materials, thermal stability

is limited by the decomposition or melting temperature of

the impregnating agents used. The temperature limit for

resin-impregnated materials is >200°C depending on the

resin used.

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Strength

Carbon graphite and graphite materials have a compa-

ratively low tensile and flexural strength, but a higher

compressive strength. In contrast to plastics or metallic

materials, their strength does not decrease with increasing

temperature.

In constructions with carbon graphite and graphite materi-

als, typical ceramics must be considered to have a certain

brittleness. Because of this greater brittleness compared to

conventional metallic materials, the strength of these mate-

rials is not to be characterized by details of tensile strength

and elongation values. Rather, it is customary to provide as

parameters the bending and compression strength and the

modulus of elasticity.

Carbon graphite is superior to electrographite in terms of

strength. Electrographite, on the other hand, has a some-

what lower brittleness. By impregnation with synthetic

resins or metals, strength, elastic moduli and hardness can

be significantly increased.

Hardness

Schunk is measuring the hardness values HR5/40, HR5/100

and HR5/150 for carbon materials. A 5-mm steel ball with

98 N preload and 294 N, 883 N or 1373 N additional load

is pressed into the body to be tested. After removing

the additional load, the remaining penetration depth is a

measure of the hardness HR5/40, HR5/100 of HR5/150

(dimensionless), which is read according to the B-scale of

Rockwell hardness testers. To facilitate comparison with the

hardness values of other materials, we have included the

Brinell Hardness in our „Characteristics - Standard materials“

brochure (30.14) as well as the Rockwell Hardness (RH).

For continuous quality control we do not measure hardness

using the Brinell method, since this is permissible only if the

surface of the porous material is polished. Dynamic hard-

ness measuring methods are in our experience less suitable

because of the structure of the material. Moreover, the use

of Shore hardness values alone is problematic because the

measured values are heavily dependent on the device used.

Thermal conductivity

In Table 1, the typical thermal conductivities of carbon

graphite and electrographite compared to other common

materials are summarized. Carbon graphites achieve the

conductivity of stainless steels, and electrographites are

distinguished by even higher thermal conductivities.

Coefficient of thermal expansion

Another important feature that must not be forgotten in

design using carbon materials is low coefficient of thermal

expansion in comparison to metals. With coefficient of

thermal expansion in comparison from 2 to 6 * 10-6/K it is

smaller than that of metals by factors.

Material Thermal Conductivity+20 °C W/m*K

Electrographite 40-130

Carbon graphite 8-17

18/8 Chrome-Nickel Steel 15

Grey Cast Iron 45-60

Copper 395

Bronze SnBz 12 38

Cast Chrome Steel 19

Sintered Ceramic (Al2O3) 21

Silicon Carbide 80-130

Table 1: Thermal Conductivity



Thermal shock resistance

Thermal shock resistance is excellent for carbon graphite

and particularly for electrographite materials. It can be

defined as the quotient of the product of strength and

coefficient of thermal expansion and the product of Young‘s

modulus and thermal expansion coefficient.

Sliding properties

Graphite, whether natural graphite or electrographite, has

self-lubricating properties by virtue of its special crystal

structure. As graphite is always used as a component in

the production of carbon graphite materials for bearings

and sealing elements, an important part of these materials

consists of dry lubricant, as well as electro-graphite. Even

without additional liquid lubricant, the friction coefficient

between carbon materials and the counter material with

perfect sliding surfaces is relatively low. Generally, valid

information on the friction coefficient cannot be determined

due to the widely differing operating conditions. In dry run-

ning with gray cast iron or steel, a friction coefficient in the

region of μ = 0.1 to 0.3 can be expected. In the presence of

liquids or vapors, whereby the type of liquids or vapors is of

minor importance, the friction coefficient is reduced signifi-

cantly in the mixed friction area to μ < 0.1. Indicators on the

course of the friction coefficient between carbon graphite

and gray cast iron or steel in dry operation are provided in

the following four diagrams.

10 20 30 40 50 60 70 min. 90 Runningtime

0.3

0.2

0.1

Coe�

cien

t of f

rictio

n µ

1 2 3 4 5 6 7 8 m/s

Mean sliding speed

0.3

0.2

0.1

Coe�

cien

t of f

rictio

n µ

0.4

0.6

0.5

10

Graph 1:

Variation of the coefficient of friction μ during running-in

Graph 2:

Coefficient of friction μ as a function of the mean sliding

speed

Test conditions:Thrust bearingCarbon grade: FH44YDiameter of carbon ring: 80.5/57.5Face area: 25 cm2

Counterface material: Cast iron (fine finished)Temperature at the running surface: ~ 100 °CSpecific loading: 1N/mm2 Mean sliding speed: 0.8 m/s

Test conditions:Thrust bearingCarbon grade: FH44YDiameter of carbon ring: 80.5/57.5Face area: 25 cm2

Counterface material: Cast iron (fine finished)Temperature at the running surface: ~ 100 °CSpecific loading: 1N/mm2

09

10

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Specific loading

0.3

0.2

0.1

Coe�

cien

t of f

rictio

n µ

0.4

0.6

0.5

1.0N/mm2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 N/mm2

Specific loading

0.3

0.2

0.1

Coe�

cien

t of f

rictio

n µ

0.4

0.6

0.5

1.0

Graph 3:

Coefficient of friction μ as a function of the specific loading

Graph 4:

Coefficient of friction μ as a function of specific loading of

the material combination FH44Z2 – steel

From: Technische Hochschule Darmstadt: Thesis H. Hart-

mann: „On the temperature variation and the limits of use

of dry-running graphite sealing rings.“-

The first diagram, in which the friction coefficient is plotted

over the run time, shows that this decreases with the

progress of the running-in and an associated progressive

smoothing of the sliding surface, and then stabilizes at a

low level. Of far greater importance however is the fact that

the friction coefficient is dependent on the sliding speed

and the specific load. Tables 2 and 3 show this dependency

for the carbon graphite material -FH44Y-.

Table 4 shows the dependence of the friction coefficient on

the specific load at two constant running speeds using the

example of the resin-impregnated carbon graphite material

-FH44Z2-.

Of particular note is that the carbon materials exhibit excel-

lent resistance to wear at low friction coefficients too.

Test conditions: Thrust bearingCarbon grade: FH44YDiameter of carbon ring: 80.5/57.5Face area: 25 cm2

Counterface material: Cast iron (fine finished)Temperature at the running surface: ~ 100 °CMean sliding speed: 0.8 m/s

Test conditions:Carbon sealing-ringCarbon grade: FH44Z2Diameter of carbon ring: 180/200 x 20mmCounterface material: Steel St60Roughness of material surface Rt < 1μm v= 5.7 m/s v= 8.6 m/s



Design notes for fine-grain carbon machine parts

Since all sliding elements made by Schunk Carbon Technology are produced according to customer drawings and/or customer specifications, the designer is not bound by standards or standard norms of dimensions or materials.

When designing bearings and sealing elements, the ceramic

properties of carbon graphite and graphite materials descri-

bed above are to be observed.

It is therefore advisable to contact us during the new com-

ponent design stage to avoid designing components that

are inconvenient or impossible to produce.

The geometries are machined typically from pressed semi-

finished products. Virtually all machining methods can be

used: e.g. sawing, water jet cutting, turning, milling, drilling,

grinding, honing, lapping and polishing. The wall thickness

should not be less than 3 mm, if possible. For round parts,

the wall thickness should be 10-20% of the inner diameter,

depending on the size of the components. The length of

the components can be up to double the outside diameter.

Furthermore, a division into two or three pieces may have to

be performed. Deep and narrow bores should be avoided. As

a rule, an inside diameter tolerance of IT7 and an external

diameter tolerance of IT6 can be complied with. Because

of the risk of fracture, it is advisable to refrain from large

cross-sectional changes. An alternative would be to divide

the part into several pieces with different wall thicknesses.

Sharp edges should be chamfered.

If components made of carbon graphite or graphite materi-

als must be mounted in metallic casings or secured against

rotation, screws and wedges are eliminated due to the

notch effect. The first choice should be a press- or shrink-fit.

If this is insufficient, pinning can be used. It is important to

ensure that this happens in an unloaded area and the car-

bon component undergoes no tensile stress by the thermal

expansion of the pin. Carbon moldings are to be fastened

in metal frames or directly in the housing when pressing or

shrinking, preferably over the entire length to support it. In

cantilever installations correspondingly large wall thicknes-

ses should be used.

D

L

s

L ≥ 3 mm

L ≤ 2 D

s ≥ 3 mm

Sub-divided into several plain parts

Protection against rotation with plain pin in the unloaded part of a carbon body

Transitions radiused, edges broken

Carbon parts shrunk or pressed into metal brush or directly into housing

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Resin bonded carbon materials

These materials consist of carbon and/or graphite-filled phenolic resin. Different compositions reflect the different requirements in respective applications.

A great advantage of these materials is the possibility of

applying plastic molding processes, which enable cost-

effective production in large quantities. In addition to the

injection molding process at Schunk, injection-compression

molding and the unidirectional pressing applied in tempered

dies are also used for these materials. Since the tool costs

of injection molding are considerable, suitable areas of

application include bearings, sealing rings and pump parts,

which are needed, for example, in the automotive industry

in large quantities. The temperature resistance up to 180°C

and the temperature expansion coefficient within the region

of metallic materials bear mentioning here. Thereby, it is

possible to injection-mould around inserts or the material

itself as an insert part. Applications at up to 250°C have al-

ready been achieved, and even the production of all-carbon

materials is possible.

The minimum wall thickness is determined primarily by the

tooling and can be as low as 0.5 mm. The maximum wall

thickness of a component should not exceed 10 mm, other-

wise efficient production can no longer be achieved due to

the longer curing times of these thermoset materials in the

heated mould cavity. Within the tool, a tolerance of IT9 to

IT10 can be used as a reference value. For measurements

in mould parting system of the tool a tolerance of ≈ 0.1 mm

can be maintained. All tolerances are dependent on nu-

merous parameters, such as material, component geometry,

number of mould cavities and thermal post-treatments.

The definite tolerances should be determined after the first

production trials via statistical analysis. For compliance with

important functional tolerances, process control methods

are used as a quality assurance measure.


Schunk Kohlenstofftechnik GmbH

Rodheimer Strasse 59

35452 Heuchelheim ¬ Germany

Phone +49 641 6080

Fax +49 641 6080 1223

E-Mail [email protected]

www.schunk-group.com

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